Multimode Fiber Modal Noise Mitigation via Vibration
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Solution Overview
Problem
Tunable diode laser absorption spectroscopy (TDLAS) measurements in combustion processes face challenges with mode and speckle noise, particularly when using a Lambertian reflecting surface, which reduces signal levels and complicates the identification of absorption spectra due to time-dependent undulating waves in the wavelength spectrum.
Innovation Solution
The use of a multimode optical fiber with an averaging component that cyclically manipulates the light distribution, such as through twisting, stretching, or vibrating the fiber, to average modal noise induced signal level variations, thereby mitigating speckle noise and improving signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a Lambertian reflecting surface is used to minimize alignment sensitivity, then alignment robustness is improved, but signal level and measurement precision deteriorate due to scattered light and speckle noise
Solution Approach 1:
The patent introduces dynamic elements (vibration mechanism, rotating diffuser) that continuously change the optical path and scattering patterns. This dynamic approach transforms the static speckle pattern into a time-averaged uniform distribution, resolving the contradiction between using a Lambertian surface for alignment robustness and maintaining measurement precision by eliminating speckle noise through temporal averaging
Solution Approach 2:
The patent specifically employs mechanical vibration of the optical fiber or diffuser to modulate the light distribution. This vibration causes the speckle pattern to fluctuate rapidly, and when detected by the sensor, the time-averaged effect produces a uniform intensity distribution, thereby eliminating speckle noise while preserving the alignment robustness benefits of the Lambertian surface
2Device complexity
If laser light is reflected off a Lambertian surface to minimize optical access requirements, then device complexity is reduced, but mode noise and speckle pattern noise increase
Solution Approach 1:
The patent introduces an intermediary element (rotating diffuser or vibrating fiber) between the Lambertian surface and the detector. This intermediary dynamically redistributes the scattered light, acting as a mediator that preserves the alignment robustness provided by the Lambertian surface while eliminating the harmful speckle noise through temporal averaging at the detection point
Solution Approach 2:
The patent employs periodic action through rotating diffusers or oscillating fibers to continuously vary the optical path. This periodic modulation causes the speckle pattern to cycle through different configurations, and the detector captures the time-averaged uniform distribution, thereby reducing device complexity while eliminating mode and speckle noise through periodic redistribution of light
3Object-generated harmful factors
If single-mode fiber is used to transmit laser light, then mode noise is minimized, but alignment sensitivity increases and robustness decreases
Solution Approach 1:
The patent applies dynamic elements at the detection end (vibrating diffuser, rotating scatterer) to transform the spatial speckle pattern into a time-averaged uniform distribution. This allows the use of single-mode fiber to minimize mode noise while the dynamic averaging mechanism compensates for alignment sensitivity issues, providing robustness without sacrificing mode noise performance
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces speckle noise, as demonstrated by a 20-fold reduction in noise peaks and valleys, resulting in a smoother signal and improved accuracy in measuring combustion parameters.
Implementation Method 1
a first optical fiber of the pair being a multimode optical fiber for transmitting the laser light
Implementation Method 2
the reflecting surface is treated to provide for a Lambertian reflection
Implementation Method 3
means for averaging modal noise induced signal level variation of light propagating within the multimode transmitting fiber
Data Source
AI summary
An apparatus and methods for measuring combustion parameters in the measurement zone of a gas turbine engine. The measurement zone is defined as being between an outer casing and an engine component having a reflecting surface inside the outer casing. The apparatus comprises a laser generating a transmitting beam of light of a select wavelength and a multimode transmitting fiber optically coupled to the laser. A transmitting optic is optically coupled to the multimode optical fiber for transmitting the beam into the measurement zone. The reflecting surface is configured to provide a Lambertian reflection. A receiving optic is positioned to receive the Lambertian reflection. Means are provided in operative association with the multimode transmitting fiber for averaging modal noise induced signal level variation of light propagating within the multimode transmitting fiber.